Plastic Pipe Finder
I used this circuit once to track down the location of a plastic pipe, buried
underground. It transmits a weak RF signal, which can be picked up by a
portable AM radio from about 6 feet away.

Op Amp Based FM Transmitter - An low power FM Transmitter using an
op-amp as the audio preamp and a single transistor as the RF amplifier…
[Designed by Kamran Ahmed]

Open-loop power supply delivers as much as 1W 01/18/01 EDN-Design
Ideas....For VCRs, TVs, and other equipment that requires a standby mode,
you must supply power to a mP when other components are asleep to receive
and interpret any wake-up signal from the remote control….PDF has several
circuits, scroll down…. [Design Idea by Christophe Basso, On Semiconductor,
Toulouse, Cedex, France]

Phone FM Transmitter - This Phone FM transmitter connects in series to your
telephone line and transmits the telephone conversation over the FM band when you pick
up the telephone handset. Transmitted signal can be tuned by any FM receiver. The
circuit includes an "On Air" LED indicator and also provides a switch which can be
used to turn off the transmitter. A unique feature of the circuit is that no battery
is needed to operate the circuit since power is taken from the telephone line.

Plastic Pipe Finder - I used this circuit once to track down the location of
a plastic pipe, buried underground. It transmits a weak RF signal, which can be
picked up by a portable AM radio from about 6 feet away. …. [Designed by David A.
Johnson]

PLL FM Transmitter - This is PLL FM Transmitter using SAA1057 chip.
Transmitter can be operated from a PC through LPT port, or using a PC software as a
driver.

Prescaler with Counter - Digital frequency counter is being used for wide
range of applications. Digital frequency counter extensively uses digital circuits and
hence fairly good knowledge of digital circuits and of digital integrated circuits is
required to understand the operation of the frequency counter. However a person who is
not familiar with any electronics circuits and experiments has in written this
article.

Radio Remote Control using DTMF - Here is a circuit of a remote control unit
which makes use of the radio frequency signals to control various electrical
appliances. This remote control unit has 4 channels which can be easily extended
to 12. This circuit differs from similar circuits in view of its simplicity and
a totally different concept of generating the control signals.... [Radioland.net]

Remote control using VHF modules - A few designs for remote control
switches, using VG40T and VG40R remote control pair, are shown here. The miniature
transmitter module shown in Fig. 1, which just measures 34 mm x 29 mm x 10 mm, can be
used to operate all remote control receiver-cum-switch combinations described in this
project.... [Radioland.net]

Remote RF Xmiter - This device was built into a small (1 1/4 by 2 inch)
Radio Shack project box. It features four push buttons which can produce 2 codes each.
(Pushing a button for less than one second sends the first code, holding it for over
one second sends the second code.) [Luhan Monat - Mesa Arizona]

RF Buffer Stage - This bufferstage is intended to be used after a BF900 or
BF199 (even a BF245) oscillator. It delivers several milliwatts and is perfect to
drive a 2N2219 or 2N4427. Be sure the coils are really round and can "see" each other:
place metalplates between the coils. The supply voltage is around 12V (typ. 13,8V).

RF Preamplifier - Input needs only several mWatts (e.g. BF900) -> 300 mWatts
OUT. This schematic is a often used buffer. It's easy to build, clean and cheap.
When you want even more power, you can replace the 2N2219a by a 2N4427 or 2N3553. You
can lower the output by adding a little resistor (10..47 Ohm 1/2W) in the emitter line
of the 2N2219a.

RF-telemetry transmitter features minimal parts count - 6-Jan-05 Issue of
EDN Requirements for portable, short-range telemetry systems frequently include
low power consumption, small size, and low cost. The circuit in Figure 1 meets these
criteria and uses only three off-the-shelf ICs and a few passive components. Although
dedicated to conditioning the low-level signal a strain-gauge bridge produces, the
circuit can operate with almost any resistive transducer based on.... [Design Idea by
Francis Rodes, Eliane Garnier, and Olivier Chevalerias, ENSEIRB Talence, France]

SAA1057 PLL Synthesized FM Transmitter - The functioning of all is provided
by a microcontroller from MICROCHIP "PIC16F84" which provides support for buttons, LCD
2 lines of 16 characters and the circuit pll "SAA1057. The VCO is entrusted to the
transistor Q8 associate of his two diodes varicaps "BB109, a floor buffer Q7 separates
the VHF signal obtained in two ways, on the one hand to Q9 to enslave loop phase and
on the other hand to the Q5 and Q6 together which takes care to amplify the signal
before attacking Q11, a "BFR96" which plays the role of HF switch via a timer "NE555"
which receives information from the push and "SAA1057" ensuring HF cutoff in the event
of failure of a locking or unlocking of the pll.

Simple FM Transmitter Schematic - There is no
thrill like the thrill you get from operating equipment you have built yourself. If
you have never built a project from a magazine before, let this be your first--you'll
see how much satisfaction and fun you can have!… [Tony van Roon]

Simple RF Power Meter - Here is a simple set up which will enable them to
measure the out put power of their transmitter. All that they require is a good
multimeter which has a sensitivety of 20k ohms/4 Watts which is adequate for low power
transmitters. ....

Single Channel RC Cars TX & RX - Here is the 9 volt GS transmitter which
does not use a crystal. This clever design allows only two transistors to
generate both the carrier and the modulation. Shown below at 27MHz 40MHz and
49MHz: (added 2/05) [Reverse engineered schematic by Stepan Novotill]

Single Chip FM Transmitter - Simple to build once chip FM Transmitter that
requires only 3, 5V to operate. This transmitter links your home-entertainment
system to a portable radio that can be carried around the house and into the back
yard. For example, you can play music on the CD....(added 8/08)

Small circuit forms programmable 4 to 20 mA transmitter - 17-Apr-03
Issue of EDN One of the key challenges in the design of 4- to 20-mA current
transmitters is the voltage-to-current conversion stage. Conventional transmitters use
multiple op amps and transistors to perform the conversion function. These approaches
have been around for a long time, but they are usually inflexible, have poor power
efficiency, and have limited current compliance.... [Design Idea by Alan Li and Jeritt
Kent, Analog Devices, Bellevue, WA]

Solar Powered FM Bug - There are many miniature FM transmitter (bug)
circuits online, this one is unique in that it runs completely on solar power. No
battery is required. As long as the sun is shining on the PV panel, the circuit will
transmit. The bug is useful as a "remote ear", and can be used for anything from
listening birds to surveillance work. The mic preamp and oscillator circuits were
borrowed from a common circuit found around the Internet, I added a regulated solar
power supply and an RF amp that extends the range and improves frequency stability....
[G. Forrest Cook]

Spark gap Transmitter - The first thing I want to stress is that "SAFETY WAS
NOT A MAJOR CONCERN" during the days of Spark, some of the methods used were VERY
DANGEROUS. Now that I have said that, let me move on. There was a certain amount of
excitement that was assoicated with Spark Gap transmitters. Without getting into the
theory or design of these Spark Gap Wireless transmitters, let me attempt to give you
an over-view of their operaton.... [Designed by w1fji]

Surveillance Transmitter Detector - This circuit can be used to "sweep" an
area or room and will indicate if a surveillance device is operative. The problem in
making a suitable a detector is to get its sensitivity just right, Too much
sensitivity and it will respond to radio broadcasts, too little, and nothing will be
heard. [Andy Collison]